Unsuitable cell exclusion in wireless communication systems

ABSTRACT

A method and system for performing initial cell search in wireless communication system wherein unsuitable cells are excluded is disclosed. Stored frequencies are searched exhaustively and initial frequencies are search non-exhaustively. Initial frequencies may be searched exhaustively in certain circumstances. When performing exhaustive initial cell searches, primary synchronization codes that lead to unsuitable cells are excluded from subsequent initial cell searches performed on the same frequency.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims priority from U.S. provisional application No.60/442,087, filed on Jan. 23, 2003, which is incorporated by referenceas if fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communication systems. Morespecifically, the present invention relates to initial cell search insuch systems.

BACKGROUND

FIG. 1 illustrates a wireless communication system 10. The communicationsystem has a plurality of base stations 12 ₁-12 _(n) (12). Each basestation 12 communicates with wireless transmit/receive units (WTRUs) 14₁-14 _(n) (14) within its operating area or cell 16 ₁-16 _(n) (16). Whena WTRU 14 is first activated, it is unaware of its location and withwhich base station 12 (or cell 16) to communicate. The process where theWTRU 14 determines the cell 16 to communicate with is referred to as“initial cell search.”

Initial cell search (ICS) in the proposed time division duplex mode ofwideband code division multiple access, by way of example, typicallycomprises three steps. In step one, a WTRU searches for a primarysynchronization code (PSC). Each base station transmits the same PSC ina primary synchronization channel (PSCH). The PSCH may be transmitted inone or two timeslots depending on the manner in which the system isimplemented. In FIG. 2, the PSCH is shown being transmitted in twotimeslots 20, 22. As shown in the exploded view of timeslot 22, eachbase station's PSC is offset in time according to a particular timeoffset. The time offset, typically expressed as particular number ofsignaling units called “chips,” is provided to reduce interferencebetween secondary synchronization codes (SSCs) which are simultaneouslytransmitted along with each PSC by each base station in a system. ThePSC that is selected by the WTRU is the one for which the WTRU hasmeasured the highest power.

In step 2, the SSCs transmitted along with the selected PSC are used todetermine the timeslot offset of the detected PSC and the code group.Then, in step 3, the scrambling code and unique midamble baseidentification number are determined.

This approach to initial cell search has drawbacks. One drawback iswhere a detected PSC leads to a cell belonging to a PLMN with which theWTRU may not communicate for whatever reason (e.g. a WTRU on a firstnetwork detecting a PSC leading to a cell of a second network whereinthere is no shared-use agreement between the two networks). In thiscase, the WTRU will read the broadcast channel (BCH) of the unsuitablecell, realize that it is an unsuitable cell and begin another initialcell search. The problem is that while running subsequent initial cellsearches, the WTRU often detects the same PSC and is led to the sameunsuitable cell. This is inefficient in that the WTRU is wasting timeand resources running additional initial cell searches for PSC's thatlead to unsuitable cells and wasting additional time and resourcesreading the BCH of unsuitable cells and may eventually cause the initialcell search to fail based on a timeout.

It is therefore desirable to have a method and system for runninginitial cell search without such limitations.

SUMMARY

The present invention is a method and system for performing initial cellsearch in wireless communication system wherein unsuitable cells areexcluded. Stored frequencies are searched exhaustively and initialfrequencies are search non-exhaustively. Initial frequencies may besearched exhaustively in certain circumstances. When performingexhaustive initial cell searches, primary synchronization codes thatlead to unsuitable cells are excluded from subsequent initial cellsearches performed on the same frequency.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a conventional wireless communication system.

FIG. 2 is a frame and an exploded view of a timeslot within the frame.

FIG. 3 is a first embodiment of a method for performing initial cellsearch using unsuitable cell exclusion logic.

FIG. 4 is a second embodiment of a method for performing initial cellsearch using unsuitable cell exclusion logic.

FIG. 5 is a method for performing a non-exhaustive initial cell search.

FIG. 6 is a method for performing an exhaustive initial cell search.

FIG. 7 is a wireless communication system in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Herein, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to herein, a base station includesbut is not limited to a Node-B, site controller, access point or anyother type of interfacing device in a wireless environment.

As mentioned, WTRUs perform an initial cell search when turned on (orotherwise need to find a cell with which to communicate). WTRUstypically include a plurality of frequencies on which they are capableof communicating. WTRUs therefore set their synthesizer to a particularfrequency and run the initial cell search (ICS) process for thatfrequency. If the WTRU is not able to find a cell with which tocommunicate at a particular frequency, the WTRU will set its synthesizerto another frequency and begin searching that frequency (i.e. performingan initial cell search for the newly selected frequency).

Often times, a WTRU is able to identify a cell with which to communicateas a result of performing an initial cell search at a particularfrequency only to determine after reading the cell's broadcast channel(BCH) that the cell is unsuitable. A cell is unsuitable with respect toa particular WTRU where the cell belongs to a public land mobile network(PLMN) on which the WTRU cannot operate. Typically, a WTRU may operateon the network of its own carrier (say carrier A) and any other carrierswith which carrier A has an agreement with respect to sharing networkhardware.

The types of frequencies on which WTRUs are capable of communicating maybe broken down into two groups, stored and initial. Stored frequenciesare those frequencies that are stored in the WTRU, either directly or byway of a SIM card or some other type of removable storage medium. Thestored frequencies are frequencies where there is a higher (higher withrespect to initial frequencies) likelihood of finding a suitable cellwhile performing initial cell search. For example, as explained in theprevious paragraph, the stored frequencies may be frequencies of notonly the carrier with which the WTRU is associated (say carrier A), butalso other carriers with which carrier A has an agreement with respectto sharing network hardware. Initial frequencies are frequencies onwhich a WTRU may communicate, but there is a lower likelihood of findinga suitable cell with which to communicate. Purely by way of example, aWTRU may be capable of communicating on a hundred frequencies whereinten are stored frequencies and ninety are initial frequencies.

Generally, as explained above, there is a higher likelihood of finding asuitable cell when searching on a stored frequency. Therefore, storedfrequencies are preferably searched exhaustively. To search a frequencyexhaustively means that where the initial cell search leads to anunsuitable cell, initial cell search is run again at the same frequencywith an exclusion window around the PSC chip-offset which led to theunsuitable cell. Initial frequencies, in contrast, are preferably searchnon-exhaustively and will only be search exhaustively in certaincircumstances as explained in greater detail below.

Referring now to FIG. 3, a first embodiment of a method 100 forperforming initial cell search using unsuitable cell exclusion logic isshown. The method begins in step 102 by determining the cell selectiontype. The cell selection type is dependent upon whether the frequenciesbeing search are stored or initial. Therefore, the cell selection typeis also referred to as being either stored or initial. If the cellselection type is stored (i.e. stored frequencies are being searched),the method 100 proceeds to step 104. In step 104, an exhaustive initialcell search is performed on the stored frequencies. An exhaustiveinitial cell search is one that in situations where an initial cellsearch on a particular frequency leads to an unsuitable cell, subsequentinitial cell searches are performed at that same frequency with anexclusion window around the PSC chip-offset which led to the unsuitablecell. Further details regarding how an exhaustive initial cell search(i.e. an exhaustive search) is performed is described in connection withFIG. 6. If there is a success at any time while performing theexhaustive search, the method 100 stops 106. If there is a failure 108,the method proceeds to step 110. In step 110, the cell selection type isagain determined. The cell selection type is determined again because,as will be explained in steps 116, 118, and 120, an exhaustive cellsearch may be performed on initial frequencies in certain circumstances.In this case, the cell selection type is stored so the method 100proceeds to step 114 where all stored frequencies are excluded and thecell selection type is set to initial. Then, in step 116, anon-exhaustive initial cell search is performed on the initialfrequencies. A non-exhaustive initial cell search is where only a singleinitial cell search is run for each frequency. That is, where anunsuitable cell is found on an initial frequency, because it is only aninitial frequency, it is not worth performing subsequent initial cellsearches on that frequency. Therefore, in non-exhaustive initial cellsearch, where an unsuitable cell is detected, the WTRU sets itssynthesizer to another initial frequency and starts anothernon-exhaustive initial cell search. Further details regardingnon-exhaustive initial cell searches (i.e. non-exhaustive searches) aredescribed in connection with FIG. 5. Of course, if in step 102, the cellselection type is initial, the method 100 also proceeds to step 116.

If in step 116, there is a success, the method 100 stops in step 118. Ifnot, the method 100 proceeds to step 120. In step 120, any initialfrequencies wherein a cell was detected, but was later determined to beunsuitable (i.e. because the cell belongs to an invalid PLMN withrespect to the WTRU performing the non-exhaustive initial cell search)is marked. These frequencies are marked so that they may be searchedexhaustively because once the stored frequencies are searched, theyrepresent the best chance at finding a suitable cell. That is, once anexhaustive initial cell search has been performed on the storedfrequencies and a cell has not been found, the next best thing is toexhaustively search the initial frequencies where a cell, albeit anunsuitable one, was found while performing a non-exhaustive initial cellsearch.

Therefore, from step 120, the method proceeds to step 122. In step 122,it is determined whether there are any initial frequencies where cellsbelonging to invalid PLMNs were detected (i.e. unsuitable cells). Ifthere are no such initial frequencies, total failure is declared and themethod 100 stops in step 124. If there are such initial frequencies,they are searched exhaustively in step 104. Again if there is a success,the method 100 stops in step 106. If there is a failure (step 108) thecell selection type is determined in step 110. In this case, the cellselection type is initial so total failure is declared and the method100 stops in step 112.

Referring now to FIG. 4, there is shown another embodiment of a method200 for performing initial cell search using unsuitable cell exclusionlogic. The method begins in step 202 by setting the startup frequencyconvergence indicator to LOW. As explained, initial cell search isperformed on a particular frequency, either stored or initial. Theparticular frequency corresponds to a frequency on which it is expectedto find a cell. The cell will be transmitting at a particular frequencywith very high accuracy because of its very high quality oscillator. Inorder for a WTRU to properly run the initial cell search, the WTRU needsto actually be generating a replica of the transmitted frequency withinan acceptable degree of error. That is, base stations typically havevery high quality oscillators while WTRUs typically have inexpensiveoscillators that do not always generate the same frequency in itssynthesizer. Therefore, it is often necessary to utilize a frequencycontroller to reduce the offset in frequency between WTRUs and basestations. The initial cell search may require less stringent frequencyoffsets; however, reading the BCH may require a very limited frequencyoffset. Reduction of this offset to an acceptable degree is referred toas convergence (i.e. the point at which the WTRU can read a cell's BCH).When the startup frequency convergence indicator is set to LOW, itindicates that there is no convergence. That is, the frequency at whichthe WTRU is transmitting is not within an acceptable degree of thefrequency the WTRU set its synthesizer to. As mentioned above, thestartup frequency convergence indicator is initially set to LOW in step202.

The method 200 then proceeds from step 202 to step 204 where the cellselection type is identified. Where the cell selection type is initial,the method 200 proceeds to step 206 where the WTRU sets the firstfrequency. That is, the WTRU sets its synthesizer to the first initialfrequency (i.e. the first non-stored frequency the WTRU is capable oftransmitting at). Once the first initial frequency is set, the WTRUinitializes itself for performing a non-exhaustive initial cell searchin step 210. That is, for example, the clocks (i.e. chip clock, timeslotclock, and frame clock) and chips are reset and the buffers are flushed.Then in step 212, the WTRU performs a non-exhaustive initial cell searchand in step 214, it is determined whether the non-exhaustive searchfailed. The details of performing a non-exhaustive cell search aredescribed in connection with FIG. 5.

If the non-exhaustive initial cell search did not fail, the method 200proceeds from step 214 to step 228. Assuming the non-exhaustive searchdid not fail, the WTRU obtains a cell identifier, a chip offsetlocation, timeslot offset, and frame boundary information regarding thecell detected as a result of the initial cell search. In step 228, theWTRU performs steady state AFC, a first significant path identifier(FTC) algorithm, and reads the BCH. The steady state AFC reduces theoffset in frequency between the frequencies at which the WTRU and cellare transmitting down to a sufficient degree so that the WTRU may readthe cell's BCH. By way of example, the steady state AFC preferablyreduces this offset to approximately 100 Hz (i.e. approximately onehundred cycles per second). The FTC algorithm provides an estimate ofthe beginning of the BCH response. Once these items, as well as anythingelse required to read the BCH of the identified cell, are complete, themethod 200 proceeds to step 230 to determine if the BCH was successfullyread. The BCH may not be successfully read because of poor signalingconditions, for example.

If the BCH is not successfully read, the method 200 proceeds from step230 to step 216 and continues as explained below. If the BCH issuccessfully read, the startup convergence indicator is set to high instep 232 meaning that the convergence is sufficient to allow the WTRU toread the BCH. By reading the cell's BCH, it may be determined whetherthe cell belongs to a desirable PLMN (i.e. a PLMN on which the WTRU mayoperate). In step 234, it is determined whether the cell is suitable ornot. The determination of whether the cell is suitable or not is madebased on the PLMN ID which is obtained by reading a cell's BCH. If thecell belongs to a PLMN on which the WTRU may operate, the cell issuitable. If the cell belongs to a PLMN on which the WTRU may notoperate, the cell is not suitable. If the cell is suitable, there is asuccess and the WTRU beginning operating using that cell (step 236). Ifthe cell is not suitable, the cell ID of the cell and the frequency onwhich the cell was found are stored in step 238, and the method 200proceeds to step 216 and continues as explained below. It is noted thatin step 238, the cell IDs of any neighboring cells are also preferablystored so that they (along with the cell ID of the unsuitable cell) maybe added to a rejected cell list so that those cells are automaticallyexcluded (i.e. without having to read their BCH) if they are detected inany future initial cell searches.

If, referring again to step 214, the non-exhaustive initial cell searchfailed, the method 200 proceeds from step 214 to step 216 to determineif there are any more initial frequencies to search. If yes, the WTRUsets it synthesizer to the next initial frequency (step 218) and returnsto step 210 and continues as described above. If there are no moreinitial frequencies to search, the method 200 proceeds from step 216 tostep 220 to determine if any initial frequencies were stored as a resultof getting a hit on a PSC that led to an unsuitable cell (i.e. a cellwith an undesired PLMN ID as explained above). If no, there is noservice currently available and the method 200 ends at step 222. If yes,the method 200 proceeds to step 224 wherein the method 200 is directedto step 240 so that an exhaustive search may be performed for thisfrequency.

Referring again to step 204, if the cell selection type is stored themethod 200 proceeds from step 204 to step 242 where the WTRU sets itsynthesizer to the first stored frequency. Then, in step 244, the cellselection type is again determined. Note, this is necessary because asexplained above, the method 200 may have proceeded to step 242 and then244 as a result of not only step 204, but also step 224.

If the cell selection type is initial, the WTRU initializes itself instep 246, for example, by preferably performing the following steps:

-   -   setting the timer(s) (i.e. chip counter, timeslot counter, etc.)    -   setting the AGC gain indicator to LOW (i.e. indicate that there        has not been any detection yet and the AGC gain indicator is not        fixed yet)    -   clearing the rejected chip offset list    -   clearing the rejected cell ID list    -   putting the rejected cell ID, and preferably the cell IDs of its        neighbors, into the rejected cell ID list    -   setting the rejected offset counter to zero    -   setting the rejected cell ID counter to one    -   setting the highest gain setting to maximize probability of        detecting a PSC    -   resetting the chip counter

If the cell selection type is stored, the WTRU initializes itself instep 248, for example, by preferably performing the following steps:

-   -   setting the timer(s) (i.e. chip counter, timeslot counter, etc.)    -   setting the AGC gain indicator to LOW    -   setting the rejected offset counter to zero    -   setting the rejected cell ID counter to zero    -   clearing the rejected chip offset list    -   clearing the rejected cell ID list    -   setting the highest gain setting to maximize probability of        detecting a PSC    -   resetting the chip counter

Once the initialization is complete (either initial or stored), themethod 200 proceeds to step 250 wherein an exhaustive initial cellsearch is performed. The details of performing an exhaustive cell searchare described in connection with FIG. 6.

In step 252, it is determined whether the exhaustive initial cell searchfailed or not. If it did not, the method 200 proceeds from step 252 tostep 256. In step 256, steady state AFC algorithm, FTC algorithm, andpossibly others (as known to those skilled in the art) are run and theBCH of the cell in the initial cell search is read. In this case, it isnecessary to reflect FTC changes to rejected chip offsets. It isnecessary to reflect the FTC changes because an FTC algorithm determinesthe beginning of the channel impulse response based on the chip-offsetmovements with respect to frequency offset, or changes due to channelconditions and advances or delays at the beginning of the channelimpulse response through the main clock which effects the counters (i.e.the chip counter, timeslot counter, etc.).

Next, in step 258, it is determined whether the BCH was successfullyread. If the BCH was successfully read, the method 200 proceeds to step260 wherein the startup frequency convergence indicator is set to highand the AGC gain indicator is also set to high. Setting the startupfrequency convergence indicator to high means that there is a sufficientdegree of frequency convergence to read the BCH and that the BCH wasread. The significance of setting the AGC gain indicator to highindicates that the gain setting is acceptable.

From step 260, the method 200 proceeds to step 262 wherein it isdetermined whether the cell is suitable or not. If the cell is notsuitable, the method 200 proceeds form step 262 to step 264. In step264, the cell ID and current chip-offset are added to a rejected cell IDlist and rejected chip-offset list, respectively. It is noted that thechip-offset of a rejected cell is the chip at which the PSC leading tothe unsuitable cell is found within a frame for the frequency on whichthe exhaustive initial cell search is being run. It is noted that instep 264, it is preferable to not only add the cell ID of the unsuitablecell, but also the cell IDs of the unsuitable cell's neighbors. This isbecause typically the neighbors of an unsuitable cell are alsounsuitable and belong to the same PLMN. Also, in step 264, the rejectedchip-offset counter and rejected cell ID counter are incremented. Fromstep 264, the method 200 goes back to step 250 and continues asdescribed above. If the cell is suitable, there is a success and themethod 200 ends in step 266.

Referring again to step 258, if the BCH was not read successfully, themethod 200 proceeds from step 258 to step 268. Referring again to step252, if the exhaustive cell search did fail, the method 200 alsoproceeds to step 268. In step 268, the frequency at which the exhaustiveinitial cell search was performed is excluded. Where initial cell searchhas failed at a particular frequency, it is obvious that that frequencyshould be excluded. However, it is important to note that where the BCHcannot be read successfully, the frequency should also be excluded. Thisis because in the exhaustive cell search procedure of the presentinvention, any detected cells are detected at the highest peak.Therefore, if the BCH cannot be read with the detected highest peak,there is no point in further searching for subsequent peaks with lessenergy in that frequency and the frequency should therefore be excluded.

From step 268, the method 200 proceeds to step 270. In step 270, it isdetermined whether there are any additional stored frequencies tosearch. If there are additional frequencies, the WTRU's synthesizer isset to the next stored frequency in step 272 and the method 200continues at step 244 as explained above. If there are no additionalfrequencies, the method 200 proceeds from step 270 to step 274.

In step 274, the cell selection type is determined. If it is initial(i.e. an initial frequency that ended up being stored because of a hitto an unsuitable cell), there is a failure and no service is currentlyavailable and the method 200 ends in step 276. If the cell selectiontype is stored, the cell selection type is set to initial in step 278and the method 200 jumps to step 208 in step 280.

Referring to FIG. 5, there is shown a method 400 for performing anon-exhaustive initial cell search. The method 400 begins in step 402with initialization of all steps of initial cell search (i.e. steps 1,2, and 3 of initial cell search). Then, in step 404, the chip counter isreset. Next, in step 406, step 1 of initial cell search is run forpreferably four frames. Of course, it may be run for any number offrames as desired. As previously explained, in step 1 of initial cellsearch, the WTRU is looking for the PSC correlator peak location withthe highest power.

In step 408, it is determined whether there is a detection of a PSC ornot. If there is a detection of a PSC, the method 400 proceeds to step410 wherein it is determined whether the startup frequency convergenceindicator is set to high or low. If high, the method 400 proceedsdirectly from step 410 to step 412 where step 2 of initial cell searchis run for eight frames. Again, step 2 may be run for any number offrames, as desired. As previously explained, in step 2 of initial cellsearch, the SSCs transmitted along with the detected PSC are used todetermine the timeslot offset of the detected PSC location and the codegroup. If, in step 410, the startup frequency convergence indicator islow, the method 400 proceeds from step 410 to step 414. In step 414, thevoltage controlled oscillator (VCO) of the WTRU running the search isinitialized to the last known value from the WTRU's previous successfulinitial cell search or from the last known best value. Then, in step416, a startup AFC algorithm is run for preferably twenty-four frames toreduce, preferably to 2 kHz, any offset between frequencies in the WTRUand base station. From step 416, the method 400 proceeds to step 412where, as mentioned, step 2 of initial cell search is run for eightframes.

From step 412, the method 400 proceeds to step 418 to determine if therehas been a detection of the SSCs. If not, there is a failure (step 420).If there is a detection, step 3 of initial cell search is run for fourframes (step 422). As previously explained, in step 3 of initial cellsearch, the scrambling code and unique midamble base identificationnumber are determined. In step 424, it is determined whether there was adetection while running step 3 of initial cell search. If there was adetection, the method 400 ends in step 426. In there was no detection,step 3 is run for another four frames in step 428. In steps 424 and 428,four frames is preferred, but step 3 may be run over any number offrames as desired. If there is a detection (step 430), the method 400ends in step 426. If not, there is a failure and the method 400 ends instep 420.

Referring again to step 408, if there is no detection in step 1, themethod 400 proceeds from step 408 to 432. In step 432, it is determinedwhether there are any gain settings left. If there are no gain settingsleft, there is a failure and the method 400 ends in step 420. If thereare additional gain settings left, the method 400 proceeds to step 434where the automatic gain controller (AGC) of the WTRU is set to the nextgain setting. By way of explanation, the gain setting is the amount ananalog signal is amplified before being put into an analog-to-digitalconverter. There are typically four gain settings and the one that isused first is typically the highest. Therefore, in step 434, theautomatic gain controller (AGC) will preferably be set to the nextlowest gain setting. Once the AGC is set to the next gain setting, themethod 400 proceeds to step 404 and continues as described above.

Referring now to FIG. 6, there is shown a method 500 for performing anexhaustive initial cell search. The method 500 begins in step 502 withinitialization of all steps of initial cell search (i.e. steps 1, 2, and3 of initial cell search). Then, in step 504, step 1 of initial cellsearch is run, preferably for four frames. Step 1 may of course be runfor any number of frames as desired. A mask is generated for anyrejected chip-offsets. As explained, a rejected chip-offset correspondsto the location of PSCs that have previously led to unsuitable cells.

Next, in step 506, it is determined whether there has been a detectionof a PSC or not. If there has been a detection of a PSC, the method 500proceeds to step 508 where it is determined whether a startup frequencyconvergence indicator of the WTRU performing the search is high or low.If it is high, the method 500 proceeds directly from step 508 to step510 where step 2 of initial cell search is run for eight frames. Again,step 2 may be run for any number of frames, as desired. If, in step 508,the startup frequency convergence indicator is low, the method 500proceeds to step 512. In step 512, the voltage controlled oscillator(VCO) of the WTRU running the search is initialized to the last knownvalue from the WTRU's previous successful initial cell search or fromthe last known best value. Then, in step 514, a startup AFC algorithm isrun for preferably twenty-four frames to reduce, preferably to 2 kHz,any offset between frequencies in the WTRU and base station. From step514, the method 500 proceeds to step 510 where, as mentioned, step 2 ofinitial cell search is run for eight frames.

From step 510, the method 500 proceeds to step 516 to determine if therehas been a detection of the SSCs. If not, there is a failure (step 518).If there is a detection, step 3 of initial cell search is run for fourframes (step 520). While four frames is preferred, step 3 may of coursebe run for as many frames as desired. In step 522, it is determinedwhether there was a detection while running step 3 of initial cellsearch. If there was a detection, the method 500 proceeds to step 524.If there was no detection, the method 500 proceeds to step 526 whereinstep 3 of initial cell search is run for another four frames. If, instep 528, it is determined that there is still no detection, the method500 ends in failure in step 520. If, in step 528, it is determined thatthere is a detection, the method 500 proceeds to step 524.

In step 524, it is determined whether there are any rejected cell IDs.If no, the initial cell search is successful and the method 500 ends instep 525. If yes, the method 500 proceeds to step 529 to determinewhether the cell ID of the detected cell is in the rejected cell IDlist. If no, the method 500 is successful and ends in step 530. If yes,the AGC gain indicator is set to high in step 532 which indicates thatthat the gain was set properly to continue searching for other peaks atstep 1. Then in step 534 the chip-offset at which the PSC was located isadded to the rejected chip-offset list and the rejected chip-offsetcounter is incremented. From step 534, the method 500 proceeds to step504 and continues as described herein.

Another way of implementing a cell ID rejection may be utilized in step2 processing. It is known by those skilled in the art that the adjacentcells in a physical location cannot be in the same code group.Therefore, if a cell ID is rejected, it can be associated to a codegroup number. If the rejected code group numbers are listed during step2 where rejected group numbers are not tested for detection, the step 2outcome is guaranteed to generate an accepted code group number if it issuccessful. Therefore, the step 3 is guaranteed to generate a differentcell ID each time it is successful. The rejection in step 2 can makePLMN exclusion process faster.

Referring again to Step 506, if there is no detection in step 1, themethod 500 proceeds to step 536. In step 536, it is determined whetherthe AGC gain indicator is high or low. If high, the method 500 ends infailure in step 520. Where there is no detection in step 1 and the AGCgain indicator is set to high, the initial cell search is consideredfailed because of that PSC correlations cannot even be detected at theirhighest peaks. Therefore, if there is no PSC detection, there is nopoint in further continuing the exhaustive initial cell search at thatfrequency and the search is considered failed.

If the AGC gain indicator is low, the method 500 proceeds from step 536to 538 where it is determined whether there are any gain settings left.If there are no gain settings left, the method 500 has failed (step520). If there are gain settings left, the AGC is set to the next lowestgain setting in step 540. The reason why gain settings are utilized fromhighest to lowest is that, as known to those skilled in the art, if aninput signal is saturated by a high gain level, there will bedegradation in detection performance; however, a signal may still bedetected. If the signal degradation due to saturation is too much thatthe signal cannot be detected, a lower gain setting will reduce theamount of saturation thereby increasing the chances of being able todetect the signal.

From step 540 the method 500 proceeds to step 542 where the chip counteris reset. Then, the method 500 continues at step 504 as explained above.Of course, if a timeout occurs (step 544) while running method 500, themethod 500 is considered failed (step 520).

Referring now to FIG. 7, there is shown a wireless communication system600 in accordance with the present invention. The system 600 includes aradio network controller 602, at least one base station 604, and atleast one WTRU 606. The WTRU 606 is configured to perform exhaustive andnon-exhaustive initial cell searches as described herein.

More specifically, the WTRU 606 includes a storage medium 608 forstoring frequencies, either stored or initial. The storage medium may bea built-in memory or any type of removable storage medium or SIM card.The WTRU 606 further includes a processor 610 configured to perform themethods and teachings described herein. That is, generally, processor610 is configured to perform an exhaustive initial cell search on anystored frequencies. If the WTRU 606 is not able to find a suitable cellby running the exhaustive cell search on the stored frequencies, theWTRU's 606 processor 610 will perform a non-exhaustive search on theinitial frequencies. If the WTRU 606 is still not able to find asuitable cell, the processor 610 will perform an exhaustive initial cellsearch on any initial frequencies wherein an unsuitable cell wasdetected. If a suitable cell is not found by searching the initialfrequencies, service is currently unavailable. It is noted that thestorage medium 608 or memory is used to store rejected chip-offsets in arejected chip-offset list. The memory may be used to store frequencies,cell IDs, PLMN IDs, or any other information associated with performingthe teachings described herein.

Of course, it is noted that memory 608 and processor 610 may be anynumber of memories and/or processors, as desired, for performing themethods and teachings of the present invention.

It is important to note that the present invention may be implemented inany type of wireless communication system, as desired. By way ofexample, the present invention may be implemented in UMTS-FDD, UMTS-TDD,TDSCDMA, CDMA2000 or any other type of wireless communication system.Further, while the present invention has been described in terms ofvarious embodiments, other variations, which are within the scope of theinvention as outlined in the claim below will be apparent to thoseskilled in the art.

1. A wireless transmit/receive unit (WTRU) comprising: a memoryconfigured for storing initial and stored frequencies; and a processorconfigured to perform initial cell search wherein chip-offset locationsleading to unsuitable cells are masked so that said chip-offsetlocations are not detected when running subsequent initial cell searchesat the same frequency, perform an exhaustive initial cell search onstored frequencies; perform a non-exhaustive initial cell search oninitial frequencies where no cell is found searching the storedfrequencies; and perform an exhaustive initial cell search on initialfrequencies where no cell is found searching the initial frequencieswherein a hit to an unsuitable cell was detected when said initialfrequencies were searched non-exhaustively.
 2. The WTRU of claim 1wherein when the processor is performing an exhaustive cell search, theprocessor maintains a list of any chip-offset locations that lead tounsuitable cells and generates a mask for said chip-offset locations sothat primary synchronization codes (PSCs) in the masked chip-offsetlocations are not detected.
 3. The WTRU of claim 2 wherein the processoris configured to associate rejected cells to a code group number so thatrejected code groups are detected in step 2 of an exhaustive initialcell search thereby avoiding the performance of step 3 processing onwhat will lead to an unsuitable cell.